The mechanism controlling passivation of metal surface at tips of the corrosion tunnels formed on aluminum by anodic etching in hot aqueous chloride solutions was studied. In the hypothesized mechanism, electrolyte diffusion in the tunnels maintained the tips of all tunnels at the equilibrium potential of an electrochemical reaction involving adsorption of chloride ions; this equilibrium potential was considered to be identical to the empirical pitting potential.Applied current reduction experiments were performed in which part of the active metal surface was passivated during tunnel growth. The decay transient of the potential measured at the tunnel mouth was predicted, assuming the hypothesized equilibrium at the tip. Transport equations valid in concentrated solutions were used in all models. Agreement between experimental and predicted decay time constants was achieved. However, the chloride ion electrochemical potential decreased linearly with length during growth, while the potential measured with a reference electrode at the mouth remained constant; it was concluded that the chloride ion was not the sole participant in the equilibrium.Above a certain current reduction, the potential transients revealed an obstruction to mass transport in the tunnel. The critical reduction size was predicted by assuming that an amount of hydrogen gas forms during passivation which is proportional to the passivated area.Measured tunnel geometry parameters (equivalent lengths) of dead tunnels approached zero as the aluminum chloride etchant concentration approached saturation. Geometry parameters at which the tip became saturated with aluminum chloride were predicted with transport equations including no obstruction or convection effects associated with bubbles. The predicted parameters were in quantitative agreement with measurements of dead tunnels. It was concluded that, despite the large thermodynamic driving force, hydrogen does not evolve from surfaces within tunnels, except during passivation.Differential metal dissolution rates of tunnels were measured by a new technique involving a superimposed AC signal on the applied current, and SEM observation. The growth rate was constant with length, indicating that neither mass transport not charge transfer determine the reaction rate.